1. Field of the Invention
The present invention relates to a raindrop quantity sensing apparatus and a wiper control system.
2. Description of Related Art
For example, a vehicle, such as an automobile, has a wiper apparatus to wipe a windshield of the vehicle. Wiper blades of the wiper apparatus are driven by a drive force generated by a wiper motor (drive source) based on a drive command signal. Each of the wiper blades is swung forward from a stop position to a return position of a wiping range of the windshield and is then swung backward from the return position to the stop position. By repeating the above described series of wiping action of the wiper blades, raindrops on the wiping range of the windshield are wiped.
Japanese Unexamined Patent Publication Number 2000-085538 discloses one such a technique. According to this technique, a raindrop sensor is placed at a top center location of the windshield of the vehicle. More specifically, the raindrop sensor is placed on a vehicle passenger compartment side of the windshield at the location near a room mirror, i.e., a rearview mirror. A quantity of raindrops in a sensing range of the raindrop sensor on the windshield is measured at the time, at which the wiper blade reaches the sensing range, and is thereafter measured once again at the time, at which the wiper blade reaches the sensing range. Then, an amount of a change in the quantity of raindrops is computed based on these measurements. Then, the quantity of raindrops, which is measured right before the reaching of the wiper blade to the sensing range, is added to the amount of change. In this way, the quantity of raindrops is measured without a need for providing a raindrop quantity sensing prohibited time period, during which the sensing of the quantity of raindrops is prohibited.
However, when the quantity of raindrops is determined based on the measurement signal of the raindrop sensor, which is obtained while the wiper blade is amid of moving in the sensing range, the quantity of raindrops may be erroneously measured. For example, the state of raindrops in the sensing range may be disturbed by the wiper blade, or the wiped water, which is wiped by the wiper blade and is placed in the sensing range, or the adhered water, which is adhered to the wiper blade, may be erroneously additionally measured. Thus, in such a case, the quantity of raindrops fell from the sky on the sensing range may not be accurately determined.
In view of the above disadvantage, the following technique has been proposed. A time period, during which the wiper blade passes the sensing range of the raindrop sensor, is set as a raindrop quantity sensing prohibited time period based on an elapsed time from the time of sensing a leading edge of an operating signal, which is outputted at the time of actual wiping movement of the wiper blade. The remaining time period other that the raindrop quantity sensing prohibited time period is set as a raindrop quantity sensing execution time period. The measurement signal of the raindrop sensor, which is outputted during the raindrop quantity sensing prohibited time period, is not used to determine the quantity of raindrops present in the sensing range. Instead, the measurement signal of the raindrop sensor, which is outputted during the raindrop quantity sensing execution time period, is used to determine the quantity of raindrops in the sensing range. In this way, it is possible to eliminate the above-described influences encountered at the time of movement of the wiper blade through the sensing range of the raindrop sensor.
However, recently, various types of vehicles are equipped with the raindrop quantity sensing apparatus, and it has been demanded to place the raindrop sensor to the other location(s), such as a lower part of the windshield other than the top center location of the windshield of the vehicle. However, when the raindrop sensor, which is placed at the lower part of the windshield, is operated in the above-described manner, in which the raindrop quantity sensing prohibited time period is set based on the elapsed time from the time of measuring the leading edge of the operating signal, the following disadvantages may be encountered.
In general, even when a stop command for stopping the wiper motor and thereby stopping the reciprocal wiping movement of the wiper blade is received from a user of the vehicle through a wiper switch, the reciprocal wiping movement of the wiper blade cannot be stopped immediately. In such a case, the wiper motor terminates the reciprocal wiping movement of the wiper blade upon moving the wiper blade to the stop position to provide clear front view. Therefore, as long as the provision of the clear view to the user is required, it is not possible to completely synchronize the operating state/stopped state of the wiper motor with the ON-state/OFF-state of the wiper switch. Thus, in order to notify the actual operational state of the wiper motor, the wiper motor outputs the operating signal, which indicates the operating state of the wiper motor, when the wiper motor is actually placed in the operating state. Also, the wiper motor outputs a negate signal, which indicates the stopped state of the wiper motor, when the wiper motor is actually placed in the stopped state.
However, none of the operating signal and the negate signal indicates the actual operational state of the wiper motor. Specifically, it is required to avoid simultaneous outputting of the operating signal and the negate signal, which indicate the opposite states, respectively. Therefore, in order to avoid this, the wiper motor outputs the operating signal after the lapse of a certain delay time period from the time of shifting of the operational state of the wiper motor from the stopped state to the operating state. Also, the outputting of the operating signal is stopped before the shifting of the operational state of the wiper motor from the operating state to the stopped state. Furthermore, the wiper motor outputs the negate signal after the wiper motor is actually placed in the stopped state. Thus, when the sensing range of the raindrop sensor is placed adjacent to the stop position of the wiper blade, by for example, providing the raindrop sensor at the lower part of the windshield, it may happen that the wiper blade has already passed the sensing range before the outputting of the operating signal upon the shifting of the operational state of the wiper motor from the stopped state to the operating state. In such a case, it is not possible to set the time period, during which the wiper blade passes the sensing range of the raindrop sensor, as the raindrop quantity sensing prohibited time period. As a result, it is not possible to eliminate the above-described influences on the result of the determination of the quantity of raindrops at the time of passing of the wiper blade through the sensing range of the raindrop sensor. Thereby, it is difficult to accurately determine the quantity of raindrops in the sensing range of the raindrop sensor.
In order to avoid the above disadvantages caused by the time difference, it is conceivable to set the sensing range of the raindrop sensor remote from the stop position of the wiper blade. However, in such a case where the sensing range of the raindrop sensor is set remote from the stop position of the wiper blade, the sensing range of the raindrop sensor is normally placed in a vertical center part of the windshield. When the raindrop sensor is placed in such a location, the sight of the user of the vehicle is disadvantageously reduced or interfered. Furthermore, the positioning of the raindrop sensor in such a location is prohibited by the law in many countries.